1. Field of the Invention
The present invention relates to a thermally loaded component and to a process for producing a component of this type.
2. Brief Description of the Related Art
Critical components, such as for example forged rotors or tubes or cast housings for high-temperature steam power plants or components of gas turbines or other turbomachines with operating temperatures of >700° C. have to be produced from nickel-base alloys having the required mechanical and creep rupture strength at these temperatures.
First of all, at temperatures directly above the range of use for high-temperature steels, nickel-base alloys are selected which have a chemical composition enabling them to achieve the desired high-temperature properties in a simple way by solution annealing at a temperature of typically in the vicinity of 1000° C., followed by cooling.
Even higher operating temperatures require even more complex nickel-base alloys which have the required mechanical and creep rupture strength at even higher temperatures (typically >750° C.). These alloys have even more complex compositions enabling them to achieve the desired properties through the formation of stable precipitations. Precipitations of this nature are produced by a heat treatment combined with a precipitation hardening which follows the prior solution annealing and is generally carried out in a temperature range between 700 and 900° C.
Precipitation-hardenable nickel-base alloys of this type have the desired properties for applications in the temperature range >>700° C. but also have a number of drawbacks:                on account of the lack of manufacturing equipment and on account of their tendency to form cracks during the production, they cannot be produced and processed in the sizes required for large rotors, tubes or housings;        on account of the wide solidification range of the alloys, they are difficult to weld without the formation of solidification cracks, which would render them unusable (cf. for example: High Temperature Materials for Power Engineering, Liege, 24–27 September 1990, p. 1309, p. 1461, p. 1471 and p. 1481);        in particular, welding of the fully hardened material promotes the formation of cracks, on account of the low ability of the material to compensate for the differential expansions which occur during welding;        on account of the elements added, which generate the ability to withstand high temperatures as a result of precipitation reactions, the alloys are expensive.        
In the case of large components which are acted on by high temperatures in operation, such as rotors, housings, tubes or the like, there are often regions in which the operating temperature is highest and regions in which the operating temperature is well below the highest operating temperature. For such situations, it has already long been proposed for the components to be assembled (welded together) from a plurality of subsections, which consist of a material matched to the operating temperature of the particular section, in accordance with the operating temperature distribution.
For example, it is known from DE-A1 199 53 079, in order to form a component, for two parts made from high-alloy, heat-resistant martensitic/ferritic steels, austenitic steels or superalloys based on nickel, nickel-iron and cobalt, to be welded together, with at least one of the parts first of all being plated with a filler based on nickel in the joining region, then the plated material being subjected to a quality heat treatment, and finally the parts being welded together using the same filler. In an exemplary embodiment which is explained in greater detail, a first component made from IN706 (Inconel 706), for example a disk of a rotor (assembled from a plurality of disks) in the solution-annealed state is plated with the filler SG-NiCr20Nb by means of submerged arc welding with wire. Then, the plated IN706 disk is subjected to a heat treatment which is required to ensure its quality (stabilization anneal at 820±15° C., cooling to RT, precipitation hardening at 730±15° C., cooling to RT). The plated IN706 disk is then welded to a further plated disk made from the high-alloy martensitic/ferritic steel St13TNiEL, with the root layers being applied by means of TIG welding and the reinforcing layers being applied by means of submerged arc wire welding. Then, the welded component undergoes a stress-relief anneal at 610±15° C.
DE-A1 101 12 062 proposes a process for welding together two parts which are subject to different levels of thermal loading and are intended in particular for a turbomachine. The first part consists of steel and the second part consists of a nickel-base alloy. In the process, prior to welding, first of all an interlayer is applied to the second part made from the nickel-base alloy, in which interlayer the additional elements which are present in the nickel-base alloy and are responsible for the formation of cracks decrease progressively from the inside outward. The second part preferably consists of IN625 (Inconel 625). The interlayer composed of a plurality of individual layers preferably consists of IN617 (Inconel 617).
Furthermore, it is known from U.S. Pat. No. 6,152,697 to assemble a welded rotor, in the sections which are subject to different levels of thermal loading, from more than two individual elements consisting of different materials. A 12Cr steel is used for the section exposed to the highest temperature, and a 2.1/4CrMoV steel and a 3.1/2NiCrMoV steel are used for sections which are subject to lower levels of thermal loading.
The known solutions cannot be used to realize economic solutions for operating temperatures of over 750° C.